Digital three-dimensional manufacturing, also known as digital additive manufacturing, is a process of making a three-dimensional solid object of virtually any shape from a digital model. Three-dimensional printing is an additive process in which one or more printheads eject successive layers of material on a substrate in different shapes. Typically, print heads, which are similar to the printheads found in document printers, include an array of ejectors that are coupled to a supply of material. Ejectors within a single ejector head can be coupled to different sources of material or each ejector head can be coupled to different sources of material to enable all of the ejectors in an ejector head to eject drops of the same material. Materials that become part of the object being produced are called build materials, while materials that are used to provide structural support for object formation, but are later removed from the object are known as support materials. Three-dimensional printing is distinguishable from traditional object-forming techniques, which mostly rely on the removal of material from a work piece by a subtractive process, such as cutting or drilling.
A prior art three-dimensional object printing system 10 is shown in FIG. 6. In the view depicted in that figure, a platform 14, called a cart, moves in a process direction P between printing stations, such as the printing station 26 shown in the figure. Printing station 26 includes four print heads 30 as shown in the figure, although fewer or more print heads can be used in a printing station. Once the cart 14 reaches the printing station 26, the cart 14 traverses in process direction P, underneath the printing station, supported by bearings 34 which roll upon a track of precision rails 38. The precision rails 38 are cylindrical rail sections that are manufactured within tight tolerances to help ensure accurate placement and maneuvering of the cart 14 beneath the print heads 30. Linear electrical motors are provided within a motor housing 42 of the printing system 10 and are operatively connected to the bearings 34 to guide the cart 14 on the precision rails 38.
As the cart 14 passes beneath the printing station 26, ejection of material occurs in synchronization with the motion of the cart. The electrical motors in housing 42 move the cart in the process direction P in an X-Y plane that is parallel to the printing station 26 as layers of material are printed on the cart 14. Additional motors (not shown) move the printing station 26 vertically with respect to the cart 14 as layers of material accumulate to form an object. Alternatively, a mechanism can be provided to move the cart 14 vertically with respect to rails 38 as the object is formed on the top surface of the cart. Once the printing to be performed by a printing station is finished, the cart 14 is moved to another printing station or to a station for layer curing or other processing.
An end view of the prior art system 10 is shown in FIG. 5 which depicts in more detail the bearings 34 on which the cart 14 rides the rails 38. The bearings 34 of the cart 14 are positioned on the precision rails 38 in an arrangement that facilitates accurate positioning of the build platen on the cart 14. Specifically, bearings 34 are positioned at a right angle to one another on one of the rails 38 to remove 4 degrees of freedom of the cart 14, while the other bearing 34 rests on the other rail 38 to remove one more degree of freedom. A linear motor operates to move the cart 14 over an upper surface 50 of the motor housing 42. The motor has a stationary motor segment (not shown) within the motor housing 42 and a magnet 46 mounted to the underside of the cart 14. Gravity and magnetic attraction between the stationary motor segment and the magnet 46 hold the bearings 34 in contact with the rails 38.
When carts are not present underneath the print heads 30, as shown in FIG. 6, errant drips of materials can fall from the print heads 30 into the functional area 54 and produce undesired debris and contamination on the precision rails 38 and the housing 42 in the functional area 54. Also, air-borne contaminants in the environment, such as dust or other particulate matter, can fall into the functional area 54 and collect on the rails 38 and the housing 42. When these contaminants and debris are located at any interface between the bearings 34 and the rails 38, the linear velocity of the cart is disrupted and the quality of the printed object is affected. Similarly, when these materials are within the gap between the top surface 50 of the motor housing 42 and the magnet 46 (shown in FIG. 5), the magnetic attraction can be affected and enable the cart to be less constrained. Additionally, the collection of material drops on top of the motor housing 42 can also affect the dissipation of heat from the motor and cause motion quality disturbances, impacting the performance and reliability of the motor. In order to produce three-dimensional objects with acceptable quality, the motion of the cart 14 beneath the print heads 30 needs to be precise. Therefore, improvements in three-dimensional printing systems that help prevent and eliminate the contamination on the precision rails and motor housing that affects the accuracy of the placement and movement of the cart would be beneficial.